Fire in a chlorine electrolysis plant

The event affected a chlor-alkali electrolysis plant shutdown and under maintenance (SEVESO upper tiers). First an explosion and then a fire occurred on the condensate pipe of the electrolyser. Flames escape from the end of the pipe and spilled into a 75 m³ open sky tank, containing the electrolysis products (water, HCL, NaOH, Na).

INITIATING CAUSE was the formation and ignition of a flammable mixture at the outlet of this pipe.
IGNITION SOURCE was a spark related to the work on the furnace.

According to the ARIA report, a design error was the cause of the incident. The overflow of the hydrogen safeguard, consisting in a hydraulic system, ad been erroneously connected to the condensate piping.
Due to the fact that the HCl furnace was in maintenance, an excess of hydrogen under overpressure formed and escaped through the condensate piping.
This area had not been identified as ATEX zone during work on the furnace, because hydrogen was not expected to be present in the area.

Therefore, the ROOT CAUSE was a wrong design. Moreover, work permits and related assignment of temporary ATEX zone should have followed stricter criteria and a more effective risk assessment.

condensate piping

The electrolysis plant was normally function, maintenance works started nearby

The storage of epoxy is destroyed et three plastic pipelines are burned. The electrolysis unit is stopped for 24 ore.

14:25 Immediately after the explosion, the staff started the fighting of the fire by means of fire-hoses.
14:30 The fire brigade arrived and cooled the platform where the furnace was located.
15:00 Observing that the fire gains intensity when reducing the cooling effect of the water, the operator decided to stop the production and the fire stopped.
Due tot possible presence of invisible flame, the fire brigade carries out a control by thermal camera on the various pipes overhanging the tank being exposed to thermal radiation, and leave at 16:00

According to the ARIA report, the operator implemented the following actions:

1. modifications to allow the independent discharge to the atmosphere of the surplus of hydrogen.
2. assessment of the electrical equipment of the area.
3. Verification of the installation modifications history
4. Review of the HAZOP (Hazard and Operation Procedures) of the circuits of hydrogen, chlorine and hydrochloric acid .
5. Control of the hydrogen detection system of the electrolytic unit